Epidermal basal cells have a complex lipid composition, characterized by cells containing different subcellular organells. During keratinization, the lipid composition (phosphatidylcholine, triacylglycerol, sterol, sterol ester, fatty acid) changes dramatically. Kooyman (Arch. Derm. Syph., 25, 444, 1932) and Long (J. Invest. Dermatol., 54, 174, 1970) compared the lipid composition of viable epidermis with stratum corneum and observed a striking decrease in the amount of phosphatidylcholine in the latter, indicating that phosphatidylcholine is present in high concentrations at the active sites of the epidermal metabolism. Concomitantly, this decrease in phosphatidylcholine is accompanied by an increase in the concentration of triacylglycerol and fatty acid.
In the stratum corneum, where keratinization comes to completion, a well-regulated balance of the phosphatidylcholine/sterol ratio exists, and this ratio is considered as very important for the maintenance of the physiological colloidal state of the cell protoplasm. Indeed, the disturbance of this relationship definitely contributes to the process of keratinization of stratum corneum which is replenished approximately every two weeks in a mature adult. It is also well known that the softness and flexibility of stratum corneum depends partially on the moisture content of the intercellular channels in this layer. Thus it has been long sought to find ways of restoring these vital qualities of the skin when they are lost, as occurs with the natural aging process, and in cases of extreme dryness.
In Canadian Pat. No. 1,134,276 it was disclosed that phosphatidylcholine, beside playing important physiological and biochemical roles, can form a complex which binds water and thereby creates a moisture reservoir which can restore a natural moisture balance to skin.
For skin preparations, numerous compositions have been proposed based on phosphatidylcholine (commonly called lecithin). Grate, U.S. Pat. No. 3,062,721, discusses lecithin in combination with certain other ingredients, which is said to provide a synergistic effect. He further explains that lecithin is a "skin's food" which penetrates deeply so as to facilitate the penetration of various other ingredients in the composition associated with the lecithin. Oleniacz, U.S. Pat. No. 3,957,971, discloses a liposome comprising a matrix of ternary lipid mixtures of lecithin, dicetylphosphate, and a sterol. Japanese Pat. No. JP 57/209207 (1982), discusses various uses contemplated for lecithin, cholesterol, and a combination of lecithin and cholesterol with acrylic polymers to promote deeper penetration of other ingredients into the epidermis.
A combination of phosphatidylcholine and sterol has been discussed for the treatment of infections, coughs, and respiratory diseases in Reifenrath, European Pat. No. EP100964.